In process and automation technology, it is known to determine or monitor the fill level of a medium by means of the capacitive measuring method. In this method, a probe and the wall of the container, or a second probe unit, form, with the medium as dielectric, a capacitor. The capacitance of this capacitor is measured and, based on its value, the fill level is ascertained. A problem with this method is that the probe unit comes into contact with the medium and that, consequently, accretion formation can occur on the probe unit. Such accretion degrades the measuring, or, in general, prevents measurement. In the state of the art, it is known to supply the probe unit with a relatively high measuring frequency (e.g. greater than 1 MHz), in order to improve insensitivity to accretion. Disadvantageous with a high measuring frequency is that this is accompanied by a reduction of the allowable, maximum probe length. This is brought about by frequency dependent resonance effects arising on the probe, which prevent a linear measuring. It is necessary, thus, to find a compromise between a large probe length (e.g. greater than 10 m) and a good insensitivity to accretion.